Add Digits

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

Follow up: Could you do it without any loop/recursion in O(1) runtime?

Credits:Special thanks to @jianchao.li.fighter for adding this problem and creating all test cases.

Solution

public class Solution {
    public int addDigits(int num) {
        if (num == 0) return 0;
        return (num-1) % 9 + 1;
    }
}